In general, RFID systems consist of a tag or multiplicity of tags implemented to provide information such as identity, features, or characteristics of an object to which the tag is affixed, and to transmit that information via an RF signal to an RFID reader in response to an RF interrogation signal received by the tag from the reader. In most instances of supply chain tagging applications, the tag is placed on a container (e.g., a carton, a case or a pallet) for a multiplicity of the same items, in contrast to item-level RFID tagging, in which each individual item is given its own RFID tag.
The identity of perhaps other information relating to the tagged article is stored in a memory device of its tag, and is transmitted by the RFID tag to a remote interrogator, or reader, in response to a scan (or query, command or interrogation—these terms, for present purposes, meaning the same thing) from the reader when the reader is within the response range of the tag, i.e., a range suitable for RF communication between reader and tag. These operating frequencies can include frequency ranges of 902 MHz to 928 MHz for the U.S., 950 MHz to 956 MHz for Europe and 865 MHz to 868 MHz for Japan.
Thus, although the term RFID has a connotation of one-way transfer of identification information from an object (the tag) to another location, RFID systems often involve two-way communication. In its most basic form, the conventional RFID tag consists of a transponder and an antenna. Often, the RFID tag itself is referred to as a transponder. These tags are in use in a variety of applications beyond supply-chain tagging, such as tracking movable assets (e.g., as diverse as rail cars and locomotives to cattle and other animals), non-stop highway toll collection, control of access to everything from secure areas of a facility to entertainment events, vehicle registration, mobile electronic payment of services, and a host of other applications where moderate communication distances and moderate data transfer are required, notwithstanding potentially difficult environments and the high speed of tagged objects. As such, RFID tags have been developed for diverse markets including retail, homeland security, health care/pharmacy, chemical, manufacturing, transport/logistics, defense/aerospace, packaging/labeling and automotive. A majority of those applications require that the RFID tag be small in size.
RFID tags may be either passive or active. A passive RFID tag lacks an internal self-sufficient power supply, e.g., a battery, and relies instead on the incoming RF query by the reader to produce sufficient power in the tag's internal circuitry to enable the tag to transmit a response. In essence, the query induces a small electrical current in the tag's antenna circuitry, which serves as the power source that enables tag operation. A passive tag may have range and function more limited than an active tag. But the absence of a battery leads to certain advantages, primarily that a passive tag can have virtually unlimited life and be fabricated at much less cost and in considerably smaller size than an active tag, thus serving an important need to improve the efficiency and accuracy of tracking systems for commerce, security and defense. The principles of the present invention are applicable to both passive and active RFID tags, but have relatively greater impact for passive tags.
Passive RFID tags usually incorporate very simple antenna structures, principally dipoles, loops or patches, in linear or circular polarized designs, with impedance matching elements. Typically, the antenna is embedded in or attached to the structure of the tag, and the antenna port has moderate impedance typically on the order of 20 to 300 ohms. In contrast, the impedance of the tag electronics is capacitive, with a typical impedance of 5-j 350 ohms.
Over the past several years, semiconductor technology has progressed to the point at which microwave Schottky diodes can, by use of CMOS (complementary metal oxide silicon) process technology, be integrated into the IC chip along with the other component circuitry of the tag's electronics. Thus, an RFID tag operating at UHF frequencies can be constructed as a single IC chip (i.e., as a radio frequency IC, or RFIC, or application-specific IC, or ASIC) together with an antenna on the same substrate. Such tags have been previously available for operation at low RF frequencies, typically at or below 13.56 MHz.
More recently, other options have been made possible for tag antennas through advances in process technology that have produced further reduction in chip size including the size of RFICs and ASICs. But antennas and impedance matching techniques heretofore proposed for passive RFID tags remain burdened by limitations on size, efficiency, cost, bandwidth, and sensitivity to nearby objects such as the surface on which the tag is mounted.
In the specific case of an RFID system consisting of a reader (interrogator) and a tag (transponder) used to complete transactions in high speed applications such as toll collection transactions by identification of authorized vehicles passing the reader in designated lane(s) at highway speed, the system configuration poses serious engineering challenges. The RFID tag should be thin, small, of straightforward design and therefore relatively easy to manufacture, of low cost, and high performance, capable of use in potentially hostile (or at least unfriendly) operating environments where the tag may be subject to extremes of vibration, chemicals, dust, temperature and humidity and other atmospheric or ambient conditions. While the utility and use of the present invention is explained in terms of an electronic toll collection, the present invention is also of great use in all other applications of RFID as well as use in systems containing fixed and handheld readers.
The design of RFID tags requires matching the antenna impedance and load impedance, usually by a matching circuit, for maximizing the RF power from the reader's interrogation or command signal received at the tag antenna to be delivered to the RFIC with minimum loss, and thereby achieve optimum tag sensitivity. The custom integrated circuit of which the RFIC is comprised may include the voltage-doubler, analog and digital circuitry of the transponder, and memory capacity to store the software programming and data to be transmitted to the reader in response to a command, as well as other electronics as may be necessary for a particular RFID design.
A significant problem encountered in seeking to use prior art RFID tags in high speed applications, such as highway on-the-fly (vehicle non-stop) toll collection systems, is the degree of difficulty encountered to design an antenna and impedance matching circuit of reasonably practical size, in order to optimize the RF communications performance of the tag.
A resonant antenna, such as the dipole antenna that has been the antenna of choice for RFID tags, has an optimum size at about a half-wavelength of the designated RF frequency for communications between RFID reader and tag. For example, if the designated RF frequency is 915 MHz, which is typical for RFID communications, one-half wavelength is about 164 mm (millimeters). The impedance matching circuit of a dipole antenna, as well as other antenna designs used in RFID tags of previous design, has a relatively small shunt inductive impedance and large series inductive impedance. Those attributes create a prodigious, virtually impossible task to design an impedance matching circuit of practical size and high energy efficiency for a dipole antenna or other antenna design heretofore proposed for use in an RFID tag.
Generally for antenna designs, the series-matching component may be easy to implement, while the shunt matching component is very difficult to implement because the circuit ground required for the shunt component is normally not well defined and not readily available without degrading antenna performance or necessitating an impractical antenna design.